Interference attenuator



Oct. 4, 1938. H OVERACKER 2,132,044

INTERFERENCE ATTENUATOR Filed April 13, 1935 2 Sheets-Sheet l SERVICE AREA INVENTORY HORACE E. 0VERAC/(ER.

ATTORNEYS.

SERVICE AREA Oct. 4, 1938. H. E. OVERACKER 2,132,044

INTERFERENCE ATTENUATOR Filed April 15, 1955 2 Sheets-Sheet 2 PERCENT EFFECTIVENESS I I I l I l I l V 500 700 900 I IOO I300 I500 FREQUENCY IN KC.

I INVENTOR HORACE E. OVERACKER.

BY ,0 g a ZI IORNEYS.

Patented Oct. 4, 1938 UNITED STATES PATENT OFFICE INTERFERENCE ATTENUATOR Calif.

Application April 13,

8 Claims.

My invention relates to the elimination of power line noise in radio broadcast receivers, and more particularly, to a means and method which can be applied to open wire lines to attenuate a definite range of frequencies traveling thereon.

Among the objects of my invention are: To isolate selected portions of open wire line service areas in order to prevent radio interference from reaching those portions; to provide an attenuation system for open wire lines to prevent travel 'of radio interference thereon; to provide a means and method for increasing the attenuation of undesirable frequencies on open wire lines carrying relatively low frequencies; and to provide a means and method for reducing power line interference in radio reception.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawings:

Figure 1 is a diagrammatic view of a threewire power line having a branch service area supplied thereby, the branch line being provided with attenuation devices.

Figure 2 is a similar diagram showing how a certain section of a power line may be isolated from the effect of traveling radio frequencies by the use of attenuation coils.

Figure 3 is a view in elevation showing one means of attaching attenuation coils to a power conductor.

Figure 4 is a perspective view of attenuation coils inserted in a three-wire transmission line.

Figure 5 is a diagram showing the per cent efliciency of two attenuation stages on noise elimination within the broadcast band.

Figure 6 is a sectional view showing how a capacity member may be attached to the coil to increase distributed capacity and capacity to core.

Noise heard in radio receivers comes from various sources, the most important of which, is the power line. Power line noise has been a source of receiving set interference since the beginning of radio broadcasting. In many cases, interference originating in and being radiated from power lines is so strong that radio receivers cannot be used at all and other power lines have suflicient 1935, Serial No. 16,180

local noise to prevent distant reception though local reception may be good due to the high power of nearby broadcasting stations. This interference problem is one of the most troublesome the power companies have to contend with because 5 people living in localities with strong interference very justly believe they have a right to average radio reception. As yet, however, satisfactory ways of preventing such power noise have not been available.

The usual procedure is for power line inspectors to follow up individual complaints and then remedy a specific nearby noise source. As the remedy is individualistic, other complaints follow. The service thus becomes endless and expensive, and oftentimes the only solution offered is to recommend changes in the installation of the receiver.

Theonly satisfactory solution of the problem is one which will apply towards the removal of all interference from the power lines so that the remedy can be applied directly to the power transmission lines in such a manner that all of the receiving sets adjacent thereto will be protected from such interference. All power companies now fully recognize the problem as above outlined and many do all they can to prevent radio noises on their lines. The technique, however, is largely empirical and not consistently reliable.

Up to ten years ago, power transmission lines were built with no regard for possible radio noise or interference. As progress has been made in the understanding of the various causes of noise, many of the old lines have been altered with partial success. Even the new lines, however, still present several sources of radio interference which the power companies have been unable to eliminate. In the meantime, however, much can be done to the transmission lines to prevent these noises, even though generated, from reaching 0 groups of radio set users and it is with this phase of the problem that the present invention is concerned.

Noises heard in radio receivers are caused by electrical disturbances and sparking originating in insulators, transformers, and hardware associated with power lines, and from corona on conductors and around insulators. Both the corona and the spark discharges which take place in the various portions of the power system have the negative resistance characteristic of all electrical arcs and set up radio frequency oscillations which cause high frequency currents to flow along the power line. Fortunately, these currents do not @1156 radiation to any great distance, or power noise would be a much more widely spread source of interference than it is.

The radio frequency currents and voltages set up inductive and electrical fields which affect receiving antennas located within a few hundred feet of the power lines. thus extend .only a relatively short distance at right angles to the lines, the noise travels along thelines with little attenuation, a single source of noise sometimes causing interference for twenty miles or more adjacent the conductor. Furthermore, the modern distribution system introduces high voltage lines into the heart of thickly settled portions of metropolitan areas,

thus bringing the power noise originating at almost any point along the main line directly into inductive relationship with large numbers of receiving antennas. If, therefore, the problem of actually preventing the noise at all sources is deferred, at least, the noise can be prevented from entering service areas.

Thus, insulators, transformers and other noise producing components ofv a power line may be treated within the service areas, but this will not stop interference. unless the insulators oriother noisy components are treated along the'entire line. It is, therefore, very desirable from an economic standpoint to have ameans for preventing noise on untreated parts of the line from coming into the treated parts within definite service areas.

Telephone,,telegraph,. and all other open. wire lines designed to carry relatively low frequency currents have the same difiiculties, and my invention, while described as applied; to power lines, which are-the greatest offenders, is obviously applicable to any open wire, line or other line whereon such interference exists.

Broadly, the present invention comprisesiplacing inductance coils in series with allline wires at regular intervals. along a short section of the lines. Ifthese coils areplaced substantially less than about one-fourth wavelength apart, they have the eifectof reducing the attenuationof the line,.as in telephone practice. If; however, as in the present invention, the coilsare located at a distance onthe order of an odd number of onequarter wavelength of the lowest frequency to be attenuated, the. line attenuation for, that, and higher frequencies, is greatly increased. By properly. spacing the coils a very great increase in attenuation is obtained over placing the same three coils or a single large coil, atone point in the lines.

My invention also comprises the use of two tuned coils in series with each wire of a line, all In this case coils being placed. at the same. pole. the two coils in series are tuned to different frequencies within the frequency band itisdesired to attenuate, thus securing a. wider band of attenuationv than can besecured by a slngle'tuned coil.

My invention also comprises the isolation of service areas as by blockingrthe main line wires oneach side of. the service areas with spaced inductances to attenuate frequencies coming in from either direction.

By attenuating a short'section of open wire line with coils spaced on the order of one-fourth wavelength apart for the lowest frequency it is desired to stop, a high attenuation. of noise can be secured and Figures 1 and 2 show definite service'areas l and 2 isolated from a main trans- While the interferences inserted in each conductor of the line on the same pole.

In Figure 1, service area I is supplied directly .what is known in the art as a deadend pole 5 for the application of the attenuation coils A; and in this instance, I prefer to utilize two separate coils differing in their inherent capacity and tuned by their distributed capacity and by the capacity to the core to different frequencies. In Figure 3 only one conductor is shown and it is to. be understood that a similar coil assembly is, to be applied to the remaining conductors supported by the pole whether there be two, three or moreyas the invention is applicable to systems of any phase and having any number of conductors,

On the dead end pole 5 is fastened a cross arm 6 having attached thereto on opposite sides an insulator 9. The main line conductor H3 is deadended on each side of the arm in the hook ll of the respective insulators 9--9. I prefer to utilize separateeye-bolts 'l-'! for holding the insulators to eliminate shunt capacity.

'I'wo'tuned attenuation coils are usedyand each comprises an iron core l2 within the insulating frame l3, the. ends of the core being within the coil winding, the insulated frame being supported by an inverted U strap l4 and having a heavy coil l5 preferably of thesame gauge wire as the line wound around the core and spaced therefrom'. The strap I4, is firmly attached to the main line conductor I by clamps I! and one end of the coil i is attached to the strap by a connection IS, the other end of the coil being connected to the similar end of the opposite coil by a jumper passing under the cross arm. In this case, the coil is left uncovered and exposed to the elements.

Iprefer to tune the coilsto a different band in such a manner that thefrequency bands attenuated overlap, thus broadening, by the use of two such coils the total band attenuated by the coils. The tuning is preferably accomplished by adjusting the inherent capacity of the coil to where it metal piece is connected to or disconnected from the line. a

In certain other installations it may be desirableto utilize a single coil per conductor per location either mounted as shown in Figure 3, or mounted on top of the cross arm as shown in another embodiment illustrated in Figure l. Here, the coilis wound on the core as before and covered with a weatherproof cover 2|, the coil and core not being shown. The assembly, as before, is supported on. the U bar 22 which is mounted on a standard insulator and pin 24; for

example, by an appropriate clamp 25; The U 75 bar insulated from the core and connected at one end to line H), preferably through the outlet of the coil. The other end of the U bar has an insulating leg 26 upon which is mounted a surge gap comprising gap arms 21 and 28, the lower'gap arm being connected'to the U bar and the upper arm 21 being connected to the opposite emerging lead 29 of the inductance coil, which lead then drops to connect with the main conductor [0 on the other side ofthe pole. The gap between the arms 21 and 28 is adjusted so that surges which .-might cause trouble within the coil may pass around the coil without damage thereto. a

r In both of the installations, as above outlined, or in others which will be readily apparent to those skilled in the art, the coils are preferably placed on or close to the same cross arm. or pole and inserted as connecting links in discontinuities of the conductors carried by the cross arm or pole. I I

The effect of attenuating the line has been computed, assuming the resistance of the coils to be negligible, which is the case for air core coils. It has been found that theattenuation of the line with pure inductance does not continue to increase as the frequency is raised above the value at which the coils become one-fourth wavelength apart. Rather, the attenuation increases to a maximum and then decreases at approximately half a wavelength spacing to form a pass band. The attenuation thus alternately increases and decreases and a series of pass bands occurs which pass bands are very undesirable from the standpoint of stopping interference.

In order to eliminate the pass bands for the purpose of stopping noise, the radio frequency resistance of the coils is made high. A high resistance to radio frequencies and a very low 60 cycle resistance is obtained in the same coil by using an iron core. This result is obtained because the iron losses, principally eddy current losses, increase very rapidly as the frequency is increased. Experiments showed that a coil could be made with a 60 cycle resistance of .1 ohm, and yet have a resistance of over 1000 ohms at 1000 k. c. The term .attenuation coil as used herein means, therefore, a coil having a relatively low low frequency resistance and a relatively high high frequency resistance.

Figure 5 was computed from actual field strength measurements made onpole No. 15 of a 3-wire, 60 cycle power line supplying an isolated area. Two stages of attenuation were used, the coils being applied to poles No. 6, 8 and 10, the noise originating at pole No. 1. The per cent efficiency of the attenuation coils over the broadcast band was even greater than that shown by the curve, as there was a small amount of noise originating within the isolated area itself. The coils were spaced two poles apart, in this case approximately approaching one-quarter wavelength of. 600 meters, the lowest frequency it was desired to attenuate.

It is possible in most cases to choose poles closely approaching the proper spacing. However, it is to be understood that when pole spacings are not such that proper attenuation can be had, the conductors may be opened between poles by a strain insulator, and a coil such as shown in Figure 3, for example, mounted on the conductor in series therewith and bridging the discontinuity.

The attenuation coils do not materially affect the transmission of 60 cycle power because the total inductance of one section is less than 2 m. h., a section comprising two sets of spaced coils, and

the inductance per mile of two wires, for example, in the power line is about 4 m. h. Hence, one section is equivalent to lengthening the power line only half a mile, a very small percentage of the length of most lines.

From the above data, it is evident that sectional attenuation can be successfully used to stop interference travel on high voltage power lines by using one, two or more sections of attenuation as required by the severity of the noise. However, it should rarely be necessary to use more than two sections, i. e., coils at more than three consecutive positions. In this case, I prefer to make the middle coils substantially twice the inductance of either end coils. Isolating a section of power line from radio noise produced on other parts of the system by this method is exceptionally inexpensive as the number of turns of wire is small per coil and no high voltage condensers are involved.

The above described method of stopping noise along a power line by means of attenuating coils is superior to the actual application of band stop filters, for example, because of the fact that shunt condensers are exceptionally expensive and in themselves liable to create trouble in the line I claim:

1. In combination with a power line. having a break therein, a loading coil comprising an iron core, an inductance wound on said core and insulated therefrom, a conductive hanger mechanically attached to said core but electrically insulated therefrom, and means for supporting said core and inductance in defined relation to said break and by said hanger, said inductance being electrically bridged across said break.

2. In combination with a power line having a break therein, a loading coil comprising an iron core, an inductance wound on said core and insulated therefrom, a conductive U-shaped hanger the legs of which are mechanically connected to opposite ends of said core and insulated therefrom, and means for clamping said hanger to a conductor of said line at one side of said break, one end of said inductance being connected to said hanger and the other end crossing the break to close the break.

3. In combination with a power line having a break therein, a loading coil comprising an iron core, an inductance wound on said core and insulated therefrom, a metal piece disposed about and insulated from said core within said inductance, a conductive hanger mechanically attached to said core but electrically insulated therefrom, means for supporting said core and inductance in defined relation to said break from said power line by said hanger, said inductance being electrically bridged across said break, and means for electrically connecting said metal piece to said hanger.

4. In combination with a power line having a break therein and a loading coil comprising an iron core having an inductance insulated from and Wound thereabout, said inductance being electrically bridged across said break, a conductive hanger mechanically attached to but electrically insulated from said core, said hanger being positioned to support said coil from said power line in defined relation to said break, a metal piece positioned between and insulated from said core and said coil, and means for connecting said metal piece electrically to said hanger.

5. In combination with a multi-conductor power line carrying commercial frequencies, means for attenuating radio frequencies on said line xhigherdthan aipredetermined minimum comprising an attenuation coil in series-with each conductor-at a 'certain location said coil-having aresistance in the order of 0.1 ohmand' an inductance in the order of 0.24'vmil1ihenry at 1000 cycles; and a-resistance in'the orderof 1000ohms andan inductance in-the order'of 0.18 millihenry at 1000 kilocycles, and a similar attenuation coil'in series with each conductor separated from said first coil by a distance greater than-an odd :numberof quarter wavelengths of the lowest:frequency to be attenuated.

'6. In combination with a multi-conductor power line carrying commercial frequencies, :means 'for attenuating radio frequencies on said line higher than a predetermined minimum como prising iron core attenuation'coils in-series with each conductor atone; geographical location, and :additional iron-core attenuation coils in series with each conductor at another geographicallocation spaced fromthe first by a distance'of'the .orderof an odd number of quarter-wavelengths of the lowestfrequency to be attenuated eaoh of said coils having a resistance'in the order'of 0.-1 ohm and an inductancein the order of 0.24 millihenry at 1000 cycles, and a resistance in the order of 1000 ohms and an inductance :in .the

order .of 0.18 millihenry vat-1000 kilocycles.

'7. Means for attenuating radio "frequencies on an open wire line,-comprising arplurality of iron core attenuation coils .spaced:along said clineat a :distance of the order of an odd number of quarter-Wavelengths of the'lowest'frequency'to 3 beattenuated, each of said coils havingavresist- 'ance in the order of 0.1 ohm and an inductance in iron core attenuation coils spaced along Said,

line at a distance of the order of anoddnumber of quarter-wavelengths of the lowest :frequency to be attenuated, the central-coil of said group having-twice'theinduotance of either of-the end coilsin said'group, and each of said end coils having a resistance of the order of 0.1 ohmandago an inductance of the-order'of-0.24 millihenry at 1 kilocycle, and resistanceon the order of 1000 ohms andinductance onthe order of'0il'8 milli- -henry at 1000Akilocycles.

HORACE E. OVERACKER. 

